The present invention relates to a method and apparatus of detecting any minute impurities contained in a mass of fluid such as a resin flowing in a molten state or a liquid flowing while at normal, high or low temperatures.
If minute impurities are contained, e.g. in a resin used to form an extrusion-molded joint (EMJ) for a high voltage cross-linked polyethylene (XLPE) insulated cable, they may cause electrical troubles. Thus, it is necessary to detect the existence of any impurities in the resin poured into a mold to form an EMJ.
In conventional methods of detecting impurities in a resin, a predetermined amount of resin is sampled continuously and the sampled resin is extruded into a sheet 0.1-0.5 mm thick. The sheet thus formed is inspected by a laser beam transmission or reflecting method to find any impurities (particles of foreign matter of a size larger than 30-40 microns). However, only a sampled-out portion of the fluid is inspected by these methods and not the entire body of fluid.
In a method of detecting impurities in a liquid medicine, a portion of the liquid is bypassed and guided into a passageway 51 made of transparent glass as shown in FIG. 4A. A laser beam is directed at a constricted portion 51a of the glass passageway from a laser 52 toward a light collector 53. When the laser beam impinges impurities in the fluid, the beam will be scattered. Part of the light scattered sideways is received by a light receptor 54 such as a photomultiplier, which produces photoelectric signals. The magnitudes of these signals are compared with those of reference signals to estimate the size and quantity of the impurities flowing in the fluid. This method, too, is a sampling method and does not result in an inspection of the entire body of fluid for impurities.